Strange booming noises were reported across the Champlain Valley Monday night, mystifying and even scaring Vermont residents who heard them. "Extremely alarming," said Christy Morgan of Colchester, who told New England Cable News she heard a bizarre noise. "It was very violent and very intense, and only [lasted] a second."

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Morgan said the sound terrified her because she thought it was an explosion. It struck around 7 p.m., Morgan said. "[It was] like somebody had pushed their refrigerator over onto its side and it was a violent crash and the whole house shook," she remembered.

Morgan said there was no sign anything had gone wrong in the house or outside of it. Others around the Champlain Valley reported hearing strange noises too; several took to social media sites to describe the noises that were puzzling them.

"They're called cryoseisms," said Larry Becker, Vermont's state geologist.

Becker said cryoseisms, often known as frost quakes, are caused when water underground freezes rapidly and expands in the kind of sharp temperature drops we've seen recently. Soil and rock splinter and break apart, resulting in a popping sound and vibrations, Becker said. "You can feel them at the felt level if they're very close to where you are in your local area, but generally you wouldn't expect to get any kind of damage out of them," he added.

Gib Brown, who works as both a geology and physics educator and as a meteorologist at NECN partner station WPTZ-TV, said he heard a cryoseism Monday, too. "I thought a tree fell on my house," Brown recalled. "I looked outside, I looked up and down the street, couldn't see anything. It had to have been a cryoseism."

Brown likened the phenomenon to the way potholes can be formed by freezing water, except under the earth, where their invisibility adds to their mystery. Brown called the frost quakes fairly rare but not wildly unusual. "I would say the numbers this year are way above average," Brown said.

Brown agreed with Becker in saying the frost quakes are unlikely to cause damage. "It's not dangerous," Brown said of the booms. "They are pretty rare, but in the Northeast they're not all that unusual. As a matter of fact, on Christmas day night they had a large cryoseism in Toronto and hundreds of thousands of people heard and it sounded like thunder to a lot of people up there."

Christy Morgan said she is glad to know the gunshot-like blast she heard was likely nothing more than mother nature at work. "It makes a lot of sense," Morgan said.

Morgan said she'll be keeping her ears open for more cryoseisms. "I wouldn't mind hearing it again," she said, smiling.